1. Field of the Invention
This invention relates to dielectric ceramic compositions and a method of manufacturing dielectric ceramics. It relates more particularly, to compositions which contain barium titanate as a main component and are used for ceramic capacitors for example and the manufacturing method of the dielectric ceramics.
2. Description of the Prior Art
Many dielectric ceramic compositions which contain barium titanate as a main component and are used for ceramic capacitors are well known. Barium titanate has the Curie point at about 120.degree. C. and exhibits a dielectric constant at that temperature on the other hand, it does not of about have a large dielectric constant at room temperature. A material called a shifter is added to the main component to shift the Curie point to a point near room temperature, whereby the dielectric ceramic compositions are provided with a large dielectric constant at room temperature. As shifter materials, tin oxide, zirconium oxide and rare earth metal are known.
Plate-type ceramic capacitors having a main component of barium titanate and containing such a shifter have been manufactured. But recently lamination technology has improved and dielectric green sheets 30-80 .mu.m thick have become available. So-called monolithic ceramic capacitors which have a plurality of dielectric thin film layers laminated between internal electrodes have entered the electronic industry, and conventional dielectric ceramic compositions have come into use in such ceramic capacitors.
On the other hand, ceramic capacitors recently produced have a trend to miniaturization, especially monolithic capacitors which tend to contain thin layers 10-20 .mu.m thick. A capacitor of this type is exposed to an electric field intensity ten times larger than a single plate type of capacitor having a thickness of 100-10000 .mu.m. Thus, a composition having less dependence on voltage is required as compared with the plate type capacitor.
In addition, as the thickness of a dielectric layer is reduced, structural defects of ceramics are apt to be reflected in the characteristics of a capacitor and, therefore, it is required for the ceramics to have a uniform and microcrystalline particle size (grain size), and fewer pores, the pores being of a minute size.
A dielectric ceramic material containing barium titanate of a small grain size as a main component is disclosed in the Japanese Patent Application No. 18059/1981, No. 16809/1982, No. 105919/1982, and No. 196469/1982. These dielectric ceramics containing barium titanate as a main component are reduced in grain size by adding cerium oxide, or cerium oxide and barium zirconate, or neodymium oxide, to barium titanate, and firing the materials in an air atmosphere.
These dielectric ceramics with small grain sizes have a maximum dielectric constant as small as about 10000 at room temperature, and this value is smaller than that of the ceramics with large grain sizes. Thus when miniaturized, it is difficult obtain a monolithic capacitor with a large electrostatic capacitance. Further, the problem of large temperature variations of the dielectric constant remains to be solved.